Pressure sensitive adhesive sheet

ABSTRACT

A pressure sensitive adhesive sheet is provided which comprises a base material having insulation properties, an inorganic filler-containing resin layer provided at least at one side of the base material and containing an insulating inorganic filler, and a pressure sensitive adhesive layer provided as an outermost layer of at least one surface. According to such a pressure sensitive adhesive sheet, insulation properties can be ensured even when the base material undergoes carbonization and/or combustion/decomposition due to high temperatures and insulation breakdown occurs in the base material.

TECHNICAL FIELD

The present invention relates to a pressure sensitive adhesive sheet having insulation properties and preferably to a pressure sensitive adhesive sheet used for electronic components.

BACKGROUND ART

Pressure sensitive adhesive sheets used for electronic components such as printed circuit boards and batteries are required not to adversely affect the electrical characteristics of the electronic components. For example, pressure sensitive adhesive sheets attached to or in contact with conductors through which electric currents flow in electronic components are required to have insulation properties.

In addition, electronic components may generate heat during use. If the pressure sensitive adhesive sheets are denatured due to such heat generation, the electronic components may be adversely affected. The pressure sensitive adhesive sheets used for electronic components are therefore required to have heat resistance.

Patent Documents 1 and 2 disclose such pressure sensitive adhesive sheets used for batteries. These pressure sensitive adhesive sheets each comprise a base material and a pressure sensitive adhesive layer provided on one surface of the base material.

PRIOR ART DOCUMENTS Patent Documents

[Patent Document 1] JP5639 733B

[Patent Document 2] JP2011-138632A

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The base material which constitutes a pressure sensitive adhesive sheet is generally formed of a material having insulation properties, but if such a base material is exposed to high temperatures, carbonization and/or combustion/decomposition (these may be referred to as “carbonization and the like,” hereinafter) may occur to change the base material as a whole to a conductor. In this case, the pressure sensitive adhesive sheet exhibits conductivity and, in the electronic component for which the pressure sensitive adhesive sheet is used, short circuit and/or thermal runaway may occur due to the pressure sensitive adhesive sheet and a serious accident may possibly occur.

In addition, when a high voltage is applied to a part of the pressure sensitive adhesive sheet, a conductor path may be formed by carbonization and the like of that part of the base material (insulation breakdown). Also in this case, short circuit and/or thermal runaway may occur as the above and a serious accident may possibly occur.

The problems of carbonization and the like and insulation breakdown as the above due to high temperatures cannot be sufficiently prevented by conventional pressure sensitive adhesive tapes.

The present invention has been made in consideration of such actual circumstances and an object of the present invention is to provide a pressure sensitive adhesive sheet that can ensure the insulation properties even when the base material undergoes carbonization and/or combustion/decomposition due to high temperatures and insulation breakdown occurs in the base material.

Means for Solving the Problems

To achieve the above object, first, the present invention provides a pressure sensitive adhesive sheet comprising: a base material having insulation properties; an inorganic filler-containing resin layer provided at least at one side of the base material and containing an insulating inorganic filler; and a pressure sensitive adhesive layer provided as an outermost layer of at least one surface (Invention 1). It is assumed that the outermost layer as referred to herein does not include a release sheet.

In the pressure sensitive adhesive sheet according to the above invention (Invention 1), even when the base material is exposed to high temperatures to change to a conductor and a high voltage is partially applied to the base film to form a conductor path, the inorganic filler contained in the inorganic filler-containing resin layer maintains the insulation properties thereby to ensure the insulation properties of the pressure sensitive adhesive sheet itself.

In the above invention (Invention 1), the base material may preferably have flame retardancy that satisfies flame retardancy level V-0 of UL94 standard (Invention 2).

In the above invention (Invention 1, 2), the inorganic filler-containing resin layer may preferably be provided at the one side of the base material and the pressure sensitive adhesive layer may preferably be provided at a side of the inorganic filler-containing resin layer opposite to the base material (Invention 3).

In the above invention (Invention 1-3), the content of the inorganic filler in the inorganic filler-containing resin layer may preferably be 0.5-50 g/m² (Invention 4),

In the above invention (Invention 1-4), the inorganic filler-containing resin layer may preferably comprise a material obtained by curing a composition that contains an active energy ray-curable component and the inorganic filler (Invention 5).

In the above invention (Invention 1-5), a minimum voltage at which insulation breakdown occurs in the base material may preferably be 1 kV or higher (Invention 6).

In the above invention (Invention 1-6), the pressure sensitive adhesive sheet may preferably be used in an electronic component (Invention 7).

Advantageous Effect of the Invention

According to the pressure sensitive adhesive sheet of the present invention, even when the base material undergoes carbonization and/or combustion/decomposition due to high temperatures and insulation breakdown occurs in the base material, the insulation properties can be ensured.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a pressure sensitive adhesive sheet according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a pressure sensitive adhesive sheet according to another embodiment of the present invention.

FIG. 3 is a cross-sectional view of a pressure sensitive adhesive sheet according to still another embodiment of the present invention.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described.

<Pressure Sensitive Adhesive Sheet>

In an embodiment, as illustrated in FIG. 1, a pressure sensitive adhesive sheet 1A according to the present embodiment may be composed of a base material 11, an inorganic filler-containing resin layer 12 provided at one side of the base material 11, a pressure sensitive adhesive layer 13 provided at a side of the inorganic filler-containing resin layer 12 opposite to the base material 11, and a release sheet 14 provided at a side of the pressure sensitive adhesive layer 13 opposite to the inorganic filler-containing resin layer 12.

In another embodiment, as illustrated in FIG. 2, a pressure sensitive adhesive sheet 1B according to the present embodiment may be composed of a base material 11, an inorganic filler-containing resin layer 12 provided at one side of the base material 11, a pressure sensitive adhesive layer 13 provided at a side of the base material 11 opposite to the inorganic filler-containing resin layer 12, and a release sheet 14 provided at a side of the pressure sensitive adhesive layer 13 opposite to the inorganic filler-containing resin layer 12.

In still another embodiment, as illustrated in FIG. 3, a pressure sensitive adhesive sheet 1C according to the present embodiment may be composed of a base material 11, two inorganic filler-containing resin layers 12 provided at both respective sides of the base material 11, a pressure sensitive adhesive layer 13 provided at a side of one of the inorganic filler-containing resin layers 12 opposite to the base material 11, and a release sheet 14 provided at a side of the pressure sensitive adhesive layer 13 opposite to the inorganic filler-containing resin layer 12.

In the pressure sensitive adhesive sheet 1A, 1B, 1C according to the present embodiment, the base material 11 having insulation properties is provided and good insulation properties are thereby exhibited under an ordinary environment, that is, under a temperature at which carbonization and/or combustion/decomposition of the base material 11 do not occur and under an environment in which the electrical load does not cause insulation breakdown. Moreover, even when the pressure sensitive adhesive sheet 1A, 1B, 1C is exposed to high temperatures and the matrices (organic components) in the base material 11 and/or the inorganic filler-containing resin layer 12 undergo carbonization and the like, the insulation properties of the pressure sensitive adhesive sheet 1A, 1B, 1C itself are ensured because the inorganic filler in the inorganic filler-containing resin layer 12 remains as an insulating film. Furthermore, even when a high voltage is partially applied to the pressure sensitive adhesive sheet 1A, 1B, 1C to flow a current through the pressure sensitive adhesive sheet 1A, 1B, 1C in the thickness direction so that the path of the current in the base material 11 changes to a conductor and the matrix in the inorganic filler-containing resin layer 12 undergoes carbonization and the like along the path of the current, the insulation properties of the pressure sensitive adhesive sheet 1A, 1B, 1C itself are ensured because the inorganic filler exists as an insulating body in the path. Owing to these features, the occurrence of short circuit and/or thermal runaway is suppressed in an electronic component for which the pressure sensitive adhesive sheet 1A, 1B, 1C is used. In specific embodiments, the pressure sensitive adhesive sheets 1A and 1C may be preferred because the inorganic filler-containing resin layer is ensured at a position nearer to an electronic component as an attachment, object and. On alternative path of the conductor path due to insulation breakdown is less likely to be formed.

1. Base Material

In the pressure sensitive adhesive sheet 1A, 1B, 1C according to the present embodiment, the base material 11 has insulation properties. The base material 11 having insulation properties allows the pressure sensitive adhesive sheet 1A, 1B, 1C to exhibit insulation properties during use under an ordinary environment.

In the base material 11, the minimum voltage at which insulation breakdown occurs may preferably be high. For example, the voltage may be preferably 1 kV or higher, particularly preferably 2 kV or higher, and further preferably 5 kV or higher. When the voltage is 1 kV or higher, the insulation breakdown of the base material 11 is less likely to occur, and the pressure sensitive adhesive sheet 1A, 1B, 1C can have high reliability.

The base material 11 may preferably have flame retardancy that satisfies the flame retardancy level V-0 of the UL94 standard. Owing to such flame retardancy of the base material 11, denaturation and deformation of the base material 11 are suppressed even when the electronic component for which the pressure sensitive adhesive sheet 1A, 1B, 1C is used generates heat. Moreover, even if troubles occur in the electronic component and it generates excessive heat, ignition and/or burning of the base material 11 are suppressed.

The material of the base material 11 is not particularly limited, provided that it can exhibit insulation properties, and can be appropriately selected from the viewpoints of the minimum, voltage at which insulation breakdown occurs, dielectric tangent, flame retardancy, heat resistance, solvent resistance, etc. In particular, it may be preferred to use a resin film as the base material 11. Examples of the resin film include films of polyesters such as polyethylene terephthalate, polybutylene terephthalate and polyethylene naphthalate, polyolefin films such as a polyethylene film and polypropylene film, cellophane, a diacetyl cellulose film, triacetyl cellulose film, acetyl cellulose butyrate film, polyvinyl chloride film, polyvinylidene chloride film, polyvinyl alcohol film, ethylene-vinyl acetate copolymer film, polystyrene film, polycarbonate film, polymethylpentene film, polysulfone film, polyether ether ketone film, polyether sulfone film, polyether imide film, fluorine resin film, polyamide film, polyimide film, polyamideimide film, acrylic resin film, polyurethane resin film, norbornene-based polymer film, cyclic olefin-based polymer film, cyclic conjugated diene-based polymer film, vinyl alicyclic hydrocarbon polymer film, other resin films, and laminated films thereof. Among these, the polyimide film, polyetherimide film, or polyether ether ketone film may preferably be used from the viewpoints that the heat resistance is high, the minimum voltage at which insulation breakdown occurs is high, and the dielectric tangent is small. In particular, it may be preferred to use the polyimide film which exhibits high heat resistance. As used in the present description, the term “polymer” encompasses the concept of a “copolymer.”

The thickness of the base material 11 may be preferably 2 μm or more, particularly preferably 6 μm or more, and further preferably 10 μm or more. From another aspect, the thickness of the base material 11 may be preferably 200 μm or less, particularly preferably 100 μm or less, and further preferably 40 μm or less. When the thickness of the base material 11 is 2 μm or more, the base material 11 has moderate rigidity and the occurrence of curl is effectively suppressed even if curing shrinkage occurs during the formation of the inorganic filler-containing resin layer 12 on the base material 11. On the other hand, the thickness of the base material 11 being 200 μm or less allows the pressure sensitive adhesive sheet 1A, 1B, 1C to have moderate flexibility and, even when the pressure sensitive adhesive sheet 1A, 1B, 1C is attached to a surface having a height difference, the pressure sensitive adhesive sheet 1A, 1B, 1C can well follow the height difference.

2. Inorganic Filler-Containing Resin Layer (1) Composition of Inorganic Filler-Containing Resin Layer

The inorganic filler-containing resin layer 12 contains an insulating inorganic filler. When the

inorganic filler-containing resin layer 12 contains an insulating inorganic filler, even in a case in which the matrices in the base material 11 and/or the inorganic filler-containing resin layer 12 undergo carbonization and/or combustion/decomposition due to high temperatures, the inorganic filler in the inorganic filler-containing resin layer 12 remains as an insulating film. Moreover, even when the insulation breakdown occurs in the base material 11 and the matrix in the inorganic filler-containing resin layer 12 undergoes carbonization and the like along the path of the insulation breakdown, the inorganic filler exists as an insulating body in the path. Owing to these features, the insulation properties of the pressure sensitive adhesive sheet 1A, 1B, 1C itself is ensured.

The inorganic filler-containing resin layer 12 may preferably be formed of a composition that contains an organic component and an inorganic filler (this composition may be referred to as a “composition for inorganic filler-containing resin layer,” hereinafter). In particular, the inorganic filler-containing resin layer may preferably be a hard coat layer from the viewpoint of heat resistance. The hard coat layer may preferably be made of a material obtained by curing a composition that contains an active energy ray-curable component and an inorganic filler. In the pressure sensitive adhesive sheet 1C illustrated in FIG. 3, the composition may be the same or different in the two inorganic filler-containing resin layers 12.

(1-1) Active Energy Ray-Curable Component

The active energy ray-curable component is not particularly limited, provided that it can be cured by irradiation with active energy rays to exhibit desired hardness.

Specific examples of the active energy ray-curable component include a polyfunctional (meth)acrylate-based monomer, (meth)acrylate-based prepolymer, and active energy ray-curable polymer, among which the polyfunctional (meth)acrylate-based monomer and/or (meth)acrylate-based prepolymer may be preferred. The polyfunctional (meth)acrylate-based monomer and the (meth)acrylate-based prepolymer may each be used alone and both may also be used in combination. As used in the present description, the (meth)acrylate refers to both an acrylate and a methacrylate. The same applies to other similar terms.

Examples of the polyfunctional (meth)acrylate-based monomer include 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, neopentyl glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, hydroxypivalic acid neopentyl glycol di(meth)acrylate, dicyclopentanyl di(meth)acrylate, caprolactone-modified dicyclopentenyl di(meth)acrylate, ethylene oxide-modified phosphoric acid di(meth)acrylate, allylated cyclohexyl di(meth)acrylate, isocyanurate di(meth)acrylate, trimethylol propane tri(meth)acrylate, dipentaerythritol tri(meth)acrylate, propionic acid-modified dipentaerythritol tri(meth)acrylate, pentaerythritol tri(meth)acrylate, propylene oxide-modified trimethylolpropane tri(meth)aerylate, tris(acryloxyethyl)isocyanurate, propionic acid-modified dipentaerythritol penta(meth)aerylate, dipentaerythritol hexa(meth)acrylate, caprolactone-modified dipentaerythritol hexa(meth)acrylate, and other appropriate polyfunctional (meth) acrylates. These may each be used alone and two or more types may also be used in combination.

On the other hand, examples of the (meth)acrylate-based prepolymer include polyester acrylate-based, epoxy acrylate-based, urethane acrylate-based, and polyol acrylate-based prepolymers. One type of prepolymer may be used alone and two or more types may also be used in combination.

A (meth)acrylic ester polymer having an active energy ray-curable group at a side chain (referred to as an “active energy ray-curable (meth)acrylic ester polymer (A),” hereinafter) can be used as the active energy ray-curable polymer. The active energy ray-curable (meth)acrylic ester polymer (A) may preferably be obtained by reacting an acrylic-based polymer (a1) having a functional group-containing monomer unit and an unsaturated group-containing compound (a2) having a substituent group that is bonded to the functional group. Preferred examples of the unsaturated group include (meth)acryloyl group.

When the inorganic filler-containing resin layer 12 of the present embodiment is a hard coat layer, the glass-transition point after curing of the active energy ray-curable component which constitutes the hard coat layer may be preferably 130° C. or higher and more preferably 150° C. or higher, and an active energy ray-curable component of which the glass-transition point is not observed may be particularly preferred. When the glass-transition point of the active energy ray-curable component satisfies the above, the hard coat layer has excellent heat resistance, and an electronic component that includes the pressure sensitive adhesive sheet 1A, 1B, 1C provided with such a hard coat layer can have excellent performance and safety,

When two or more types of the active energy ray-curable components are used in the hard coat layer of the present embodiment., it may be preferred for them, to have excellent compatibility with each other.

(1-2) Other Organic Components

Examples of organic components that can be used other than the active energy ray-curable component include a thermoset resin and a thermoplastic resin. By compounding such components, interfacial adhesion between the inorganic filler-containing resin layer 12 and the base material 11 can be made higher. Specific examples of such organic components include a polyamide imide resin, polyether imide resin, polyimide resin, polyarylate resin, polyether ether ketone resin, polysulfone resin, melamine resin, and phenol resin. Among these, the polyamide imide resin may be preferred because of its excellent heat resistance,

(1-3) Inorganic Filler

The content of the inorganic filler in the inorganic filler-containing resin layer 12 may be preferably 0.5-50g/m², particularly preferably 0.5-10 g/m², and further preferably 0.5-2 g/m². When the content of the inorganic filler is 0.5 g/m² or more, even in a case in which the matrices in the base material 11 and/or the inorganic filler-containing resin layer 12 undergo carbonization and/or combustion/decomposition due to high temperatures, the inorganic filler in the inorganic filler-containing resin layer 12 well remains as an insulating film. Moreover, even when the insulation breakdown occurs in the base material 11 and the matrix in the inorganic filler-containing resin layer 12 undergoes carbonization and the like along the path of the insulation breakdown, the inorganic filler well exists as an insulating body in the path. Owing to these features, the insulation properties of the pressure sensitive adhesive sheet 1A, 1B, 1C itself can be ensured. When the content of the inorganic filler is 50 g/m² or less, the production cost is avoided from being unduly high and it is easy to form a layer using the composition for inorganic filler-containing resin layer. The content of the inorganic filler in the inorganic filler-containing resin layer 12 can be appropriately controlled by adjusting the addition amount of the inorganic filler in the composition for inorganic filler-containing resin layer and/or the thickness of the inorganic filler-containing resin layer 12.

When the inorganic filler-containing resin layer 12 is formed using the composition for inorganic filler-containing resin layer, the content of the inorganic filler in the composition for inorganic filler-containing resin layer may be preferably 1-99 mass %, particularly preferably 10-90 mass %, and further preferably 45-75 mass %.

The inorganic filler is not restricted, provided that it has insulation properties. Examples of the inorganic filler include powders of silica, alumina, boehmite, talc, calcium carbonate, titanium oxide, iron oxide (III), silicon carbide, boron nitride, zirconium oxide and other appropriate materials, spherical beads thereof, single crystal fibers, and glass fibers. These can each be used alone and two or more types can also be used in combination. Among these, silica, alumina, boehmite, titanium oxide, zirconium oxide and the like may be preferred. From the viewpoint of insulation properties, silica and alumina may be preferred and silica may be particularly preferred.

It may also be preferred for the inorganic filler to be surface-modified. A reactive silica can be exemplified as the preferred inorganic filler which is surface-modified.

As used in the present description, the “reactive silica” refers to silica fine particles that are surface-modified with an organic compound having an active energy ray-curable unsaturated group. The above silica fine particles (reactive silica) which are surface-modified with an organic compound having an active energy ray-curable unsaturated group may ordinarily be obtained, for example, by a reaction between silanol groups on the surfaces of silica fine particles and an active energy ray-curable unsaturated group-containing organic compound having functional groups (such as isocyanate groups, epoxy groups, and carboxy groups) that can react with the silanol groups. The average diameter of the silica fine particles may be about 0.5-500 nm and preferably 1-200 nm. Preferred examples of the above active energy ray-curable unsaturated group include a (meth)acryloyl group and a vinyl group.

Examples to be preferably used as the active energy ray-curable unsaturated group-containing organic compound having functional groups that can react with the silanol groups include compounds or the like represented by General Formula (I).

(In the formula, R¹ represents a hydrogen atom or a methyl group and R² represents a halogen atom,

Available examples of such compounds include (meth)acrylic acid and derivatives of (meth)acrylic acid, such as (meth)acrylic chloride, (meth)acryloyloxyethyl isocyanate, glycidyl (meth)acrylate, 2,3-iminopropyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, and acryloyloxypropyltrimethoxysilane. These (meth)acrylic acid derivatives may each be used alone and two or more types may also be used in combination.

Commercially-available examples of an organic-inorganic hybrid material (organosilica sol) that contains such an reactive silica and the previously-described polyfunctional (meth)acrylate-based monomer and/or (meth)acrylate-based prepolymer include products of the trade name “OPSTAR Z7530,” “OPSTAR Z7524,” “OPSTAR TU4086,” and “OPSTAR Z7537” (all available from JSR Corporation),

Other examples of preferred inorganic fillers include alumina ceramic nanoparticles, a silica sol in which silica fine particles having silanol groups exposed at the silica surface are suspended in a colloidal state in the dispersion medium, and an organosilica sol in which silanol groups on the silica surface are surface-treated with a silane coupling agent or the like.

The average particle diameter of the inorganic filler used in the present embodiment may be preferably 1-1,000 nm, particularly preferably 10-500 nm, and further preferably 20-200 nm. When the average particle diameter of the inorganic filler is 1 nm or more, the inorganic filler-containing resin layer 12 obtained by curing the composition for inorganic filler-containing resin layer has more stable insulation properties. When the average particle diameter of the inorganic filler is 1,000 nm or less, the dispers ibility of the inorganic filler in the composition for inorganic filler-containing resin layer is excellent and it is thus possible to effectively prevent the occurrence of irregularities on the surface of the inorganic filler-containing resin layer 12 opposite to the base material 11 during the formation of the inorganic filler-containing resin layer 12 on the base material 11. Moreover, when the pressure sensitive adhesive layer 13 is formed on that surface, significantly high smoothness can be obtained on the surface of the pressure sensitive adhesive layer 13 opposite to the inorganic filler-containing resin layer 12. This allows the pressure sensitive adhesive layer 13 to exhibit an excellent adhesion property to an adherend. The average particle diameter of the inorganic filler is to be measured using a zeta potential measurement method.

(1-4) Other Components

The composition for forming the inorganic filler-containing resin layer 12 of the present embodiment may contain various additives in addition to the above-described components. Examples of such additives include a photopolymerization initiator, antioxidant, antistatic, silane coupling agent, antiaging agent, thermal polymerization inhibitor, colorant, surfactant, storage stabilizer, plasticizer, glidant, antifoam, and organic-based filler.

When the inorganic filler-containing resin layer 12 is a hard coat layer, it is preferred to use a photopolymerization initiator together with the active energy ray-curable component in the hard coat layer. The photopolymerization initiator is not particularly limited, provided that it functions as a photopolymerization initiator for the active energy ray-curable component to be used. Examples of the photopolymerization initiator include acylphosphine oxide-based compounds, benzoin compounds, acetophenone compounds, titanocene compounds, thioxanthone compounds, and peroxide compounds. Specific examples include 1-hydroxycyclohexyl phenyl ketone, 2-hydroxy-2-methyl-1-phenyl-propane-1-one, 2,2-dimethoxy-1,2-diphenylethan-1-one, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzyl diphenyl sulfide, tetramethyl thiuram monosulfide, azobisisobutyronitrile, dibenzyl, diacetyl, and β-chloroanthraquinone.

The content of the above photopolymerization initiator may be preferably 0.1-20 mass parts in general and particularly preferably 1-15 mass parts to 100 mass parts of the active energy ray-curable component.

(2) Physical Properties of Hard Coat Layer as Inorganic Filler-Containing Resin Layer

In the pressure sensitive adhesive sheet 1A, 1B, 1C according to the present embodiment, when the inorganic filler-containing resin layer 12 is a hard coat layer, the pencil hardness of the hard coat layer in a state of being formed on the base material 11 may be preferably HB or more and particularly preferably H or more as measured in accordance with JIS K5600-5-4. When the hard coat layer has such pencil hardness, excellent heat resistance can be exhibited. The upper limit of the pencil hardness of the hard coat layer is not particularly restricted, but may be preferably 9H or less, particularly preferably 6H or less, and further preferably 3H or less from the viewpoint of obtaining excellent following ability to a height difference. In the pressure sensitive adhesive sheet 1C illustrated in FIG. 3, the pencil hardness of the two hard coat layers may be the same or different. The pencil hardness of the hard coat layer or layers is as described in the exemplary test, which will be described later.

(3) Thickness of Inorganic Filler-Containing Resin Layer

The thickness of the inorganic filler-containing resin layer 12 may be preferably 0.1 μm or more, particularly preferably 0.5 μm or more, and further preferably 1 μm or more. From another aspect, the thickness of the inorganic filler-containing resin layer 12 may be preferably 10 μm or less, particularly preferably 7 μm or less, and further preferably 4 μm or less. When the thickness of the inorganic filler-containing resin layer 12 is 0.1 μm or more, the insulation properties by the inorganic filler can be effectively exhibited even in cases in which the base material 11 undergoes carbonization and/or combustion/decomposition due to high temperatures and insulation breakdown occurs in the base material 11. On the other hand, the thickness of the inorganic filler-containing resin layer 12 being 10 μm or less allows the pressure sensitive adhesive sheet 1A, 1B, 1C to have moderate flexibility and, even when the pressure sensitive adhesive sheet 1A, 1B, 1C is attached to a surface having a height difference, the pressure sensitive adhesive sheet 1A, 1B, 1C can well follow the height difference. In the pressure sensitive adhesive sheet 10 illustrated in FIG. 3, the thicknesses of the two inorganic filler-containing resin layer 12 may be the same or different.

3. Pressure Sensitive Adhesive Layer

In the pressure sensitive adhesive sheet 1A, 1B, 1C according to the present embodiment, the pressure sensitive adhesive layer 13 is provided as an outermost layer (extruding a release sheet) of at least one surface. The pressure sensitive adhesive sheet 1A, 1B, and 1C illustrated respectively in FIGS. 1 to 3 each have one pressure sensitive adhesive layer 13, but may each be provided with an additional pressure sensitive adhesive layer 13. For example, in the pressure sensitive adhesive sheet 1A illustrated in FIG. 1, an additional pressure sensitive adhesive layer 13 may be added on the surface of the base material 11 opposite to the inorganic filler-containing resin layer 12. In the pressure sensitive adhesive sheet 1B illustrated in FIG. 2, an additional pressure sensitive adhesive layer 13 may be added on the surface of the inorganic filler-containing resin layer 12 opposite to the base material 11. In the pressure sensitive adhesive sheet 1C illustrated in FIG. 3, an additional pressure sensitive adhesive layer 13 may be added on the surface, opposite to the base material 11, of the inorganic filler-containing resin layer 12 located at the upper side in the figure.

(1) Composition of Pressure Sensitive Adhesive Layer

The pressure sensitive adhesive which constitutes the pressure sensitive adhesive layer 13 is not particularly limited and can be appropriately selected from the viewpoints of flame retardancy, heat resistance, insulating properties, and the like. In particular, an acrylic-based pressure sensitive adhesive, silicone-based pressure sensitive adhesive, rubber-based pressure sensitive adhesive, and urethane-based pressure sensitive adhesive may be preferred as the pressure sensitive adhesive which constitutes the pressure sensitive adhesive layer 13. Among these, the acrylic-based pressure sensitive adhesive may be particularly preferred from the viewpoints of an adhesion property to the inorganic filler-containing resin layer 12, an electrode and the like, easy delicate adjustment of the storage elastic modulus at 120° C. and adhesive strength of the pressure sensitive adhesive layer 13, etc. The storage elastic modulus at 120 ° C. of the pressure sensitive adhesive layer 13 will be described

(1-1) Acrylic-Based Pressure Sensitive Adhesive

The acrylic-based pressure sensitive adhesive may preferably contain a (meth)acrylic ester polymer and particularly preferably contain a compound in which the (meth)acrylic ester polymer is crosslinked by a crosslinker.

The (meth)acrylic ester polymer may preferably contain a (meth)acrylic alkyl ester of which the carbon number of alkyl group is 1-20, as a monomer that constitutes the polymer. This allows the obtained pressure sensitive adhesive to exhibit a preferred pressure sensitive adhesive property. The (meth)acrylic ester polymer may be particularly preferably a copolymer of a (meth)acrylic alkyl ester of which the carbon number of alkyl group is 1-20, a monomer having a functional group that reacts with the crosslinker (reactive functional group-containing monomer), and other monomers that may be used, as desired.

Examples of the (meth)acrylic alkyl ester of which the carbon number of alkyl group is 1-20 include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, n-pentyl (meth)acrylate, n-hexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, isooctyl (meth)acrylate, n-decyl (meth)acrylate, n-dodecyl (meth)acrylate, myristyl (meth)acrylate, palmityl (meth) acrylate, and stearyl (meth) acrylate. Among these, (meth)acrylic alkyl ester of which the carbon number of alkyl group is 1-8 may be preferred from the viewpoint of further improving the pressure sensitive adhesive property, and the methyl (meth)acrylate, n-butyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate may be particularly preferred. These may each be used alone and two or more types may also be used in combination.

The (meth)acrylic ester polymer may preferably contain 40-99 mass %, particularly preferably contain 50-90 mass %, and further preferably contain 75-85 mass % of the (meth)acrylic alkyl ester of which the carbon number of alkyl group is 1-20, as the monomer unit which constitutes the polymer,

Preferred examples of the above reactive functional group-containing monomer include a monomer having a hydroxy group in the molecule (hydroxy group-containing monomer), a monomer having a carboxy group in the molecule (carboxyl group-containing monomer), and a monomer having an amino group in the molecule (amino group-containing monomer). These may each be used alone and two or more types may also be used in combination.

Examples of the hydroxy group-containing monomer include hydroxyalklyl (meth)acrylate, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, and 4-hydroxybutyl (meth) acrylate. Among these, the 2-hydroxyethyl (meth)acrylate and 4-hydroxybutyl (meth)acrylate may be preferred from the viewpoints of reactivity of the hydroxy group with the crosslinker in the (meth)acrylic ester polymer to be obtained and copolymerizability with other monomers. These may each be used alone and two or more types may also be used in combination.

Examples of the carboxy group-containing monomer include ethylenically-unsaturated carboxylic acid, such as acrylic acid, methacrylic acid, crotonic acid, maleic acid, itaconic acid, and citraconic acid. Among these, the acrylic acid may be preferred from the viewpoints of reactivity of the carboxy group with the crosslinker in the (meth)acrylic ester polymer to be obtained and copolymerizability with other monomers. These may each be used alone and two or more types may also be used in combination,

Examples of the amino group-containing monomer include aminoethyl (meth)acrylate and n-butylaminoethyl (meth) acrylate. These may each be used alone and two or more types may also be used in combination.

The (meth)acrylic ester polymer may preferably contain 0.1-40 mass %, particularly preferably contain 0.5-20 mass %, and further preferably contain 1.0-4.0 mass-% of the reactive functional group-containing monomer. When the content of the reactive functional group-containing monomer is 0.1 mass-% or more, it is possible to effectively improve the cohesive strength of the pressure sensitive adhesive obtained by constituting a crosslinked structure via a cross linker which will be described later. When the content is 40 mass-% or less, a desired pressure sensitive adhesive property may readily be obtained.

Examples of the above other monomers include alkoxyalkyl (meth)acrylate, such as methoxyethyl (meth)acrylate and ethoxyethyl (meth)aerylate, (meth)acrylate having an aliphatic ring, such as cyclohexyl (meth)acrylate, non-crosslinkable acrylamide, such as acrylamide and methacrylamide, (meth)acrylate having a non-crosslinkable tertiary amino group, such as N,N-dimethylaminoethyl (meth)acrylate and N,N-dimethylaminopropyl (meth)acrylate, vinyl acetate, and styrene. These may each be used alone and two or more types may also be used in combination.

The polymerization form of the (meth)acrylic ester polymer may be a random copolymer and may also be a block copolymer.

The weight-average molecular weight of the (meth)acrylic ester polymer may be preferably 300,000-2,500,000, particularly preferably 400,000-1,900,000, and further preferably 700,000-1,600,000. When the weight-average molecular weight of the (meth)acrylic ester polymer is 300,000 or more, the durability of the pressure sensitive adhesive layer 13 is excellent. When the weight-average molecular weight of the (meth) acrylic ester polymer is 2,500,000 or less, good coating properties are obtained. As used in the present description, the weight-average molecular weight refers to a standard polystyrene equivalent value that is measured using a gel permeation chromatography (GPC) method.

In the above acrylic-based pressure sensitive adhesive, one type of the (meth)acrylic ester polymer may be used alone and two or more types may also be used in combination,

It suffices that the crosslinker is reactive with a reactive functional group of the (meth)acrylic ester polymer. Examples of the crosslinker include an isocyanate-based crosslinker, epoxy-based crosslinker, amine-based crosslinker, melamine-based crosslinker, aziridine-based crosslinker, hydrazine-based crosslinker, aldehyde-based crosslinker, oxazoline-based crosslinker, metal alkoxide-based crosslinker, metal chelate-based crosslinker, metal salt-based crosslinker, and ammonium salt-based crosslinker. One type of the crosslinker may be used alone and two or more types may also be used in combination.

The isocyanate-based crosslinker contains at least a polyisocyanate compound. Examples of the polyisocyanate compound include aromatic polyisocyanates such as tolylene diisocyanate, diphenylmethane diisocyanate and xvlylene diisocyanate, aliphatic polyisocyanates such as hexamethylene diisocyanate, alicyclic polyisocyanates such as isophorone diisocyanate and hydrogenated diphenylmethane diisocyanate, biuret bodies and isocyanurate bodies thereof, and adduct bodies that, are reaction products with low molecular active hydrogen-containing compounds such as ethylene glycol, propylene glycol, neopentyl glycol, trimethylol propane, and castor oil. Among these, trimethylolpropane-modified aromatic polyisocyanate may be preferred and trimethylolpropane-modified tolylene diisocyanate and trimethylolpropane-modified xylylene diisocyanate may be particularly preferred from the viewpoint of reactivity with hydroxy groups.

The amount, of the crosslinker to be used may be preferably 0.001-10 mass parts, particularly preferably 0.01-5 mass parts, and further preferably 1-3 mass parts to 100 mass parts of the (meth)acrylic ester polymer.

When the pressure sensitive adhesive composition which contains a crosslinker and a (meth)acrylic ester polymer that contains a reactive functional group-containing monomer as a monomer unit constituting the polymer is subjected to heating or the like, the crosslinker reacts with the reactive functional group of the reactive functional group-containing monomer which constitutes the (meth)acrylic ester polymer. This allows a structure to be formed in which the (meth)acrylic ester polymer is crosslinked by the crosslinker. The gel fraction of the obtained pressure sensitive adhesive can thus be set to a desired value, and the cohesive strength of the pressure sensitive adhesive and therefore the strength and durability are improved.

If desired, the above acrylic-based pressure sensitive adhesive may contain various commonly-used additives, such as a refractive index adjuster, antistatic, tackifier, silane coupling agent, antioxidant, ultraviolet absorber, light stabilizer, softening agent, filler, light curing agent, and photopolymerization initiator.

(1-2) Silicone-based Pressure Sensitive Adhesive

The silicone-based pressure sensitive adhesive may preferably contain an organopolysiloxane, in particular, an addition-type organopolysiloxane (a cured product thereof). The addition-type organopolysiloxane may preferably be obtained through a reaction between an organopolysiloxane of which the main skeleton is a siloxane bond and which has an alkenyl group and an organohydrogenpolvsiloxane,

The organopolysiloxane of which the main skeleton is a siloxane bond and which has an alkenyl group may preferably be a compound that is represented by the following average unit formula (II) and has at least two alkenyl groups in the molecule.

R¹aSiO_((4-a)/2)   (II)

(In the formula, each R¹ is independently the same or different unsubstituted or substituted monovalent hydrocarbon group of which the carbon number is 1-12 and preferably 1-8, and “a” is a positive number of 1.5-2.8, preferably 1.8-2.5, and more preferably 1.95-2.05.)

Examples of the above unsubstituted or substituted monovalent hydrocarbon group represented by R¹ bonded to the silicon atom, include alkenyl groups such as a vinyl group, allyl group, propenyl group, isopropenyl group, butenyl group, hexenyl group, cyclohexenyl group and octenyl group, alkyl groups such as a methyl group, ethyl group, propyl group, isopropyl group, butyl group, isobutyl group, tert-butyl group, pentyl group, neopentyl group, hexyl group, cyclohexyl group, octyl group, nonyl group and decyl group, aryl groups such as a phenyl group, tolyl group, xylyl group and naphthyl group, aralkyl groups such as a benzyl group, phenylethyl group and phenylpropyl group, and those obtained by substituting a part or all of the hydrogen atoms of these groups with halogen atoms such as fluorine, bromine and chlorine, cyano groups, or other appropriate groups, for example, a chloromethyl group, chloropropyl group, bromoethyl group, trifluoropropyl group, cyanoethyl group, and the like. The alkenyl group may preferably be a vinyl group from the viewpoint of reducing the curing time and high productivity.

The organohydrogenpolysiloxane has a SiH group in the molecule. The above alkenyl group of the organopolysiloxane and the SiH group of the organohydrogenpolysiloxane react with each other through an addition reaction and an addition-type organopolysiloxane can thereby be obtained.

The addition-type organopolysiloxane is well cured in the presence of a platinum catalyst and, therefore, the above silicone-based pressure sensitive adhesive may preferably contain a platinum catalyst. Examples of the platinum catalyst include platinum black, platinum chloride, chloroplatinic acid, a reaction product of chloroplatinic acid and monohydric alcohol, a complex of chloroplatinic acid and olefins, and platinum bisacetoacetate.

The content of the platinum catalyst in the above silicone-based pressure sensitive adhesive may be preferably 0.01-3 mass parts and particularly preferably 0.05-2 mass parts to 100 mass parts of the addition-type organopolysiloxane. The content of the platinum catalyst being within the above range allows the addition-type organopolysiloxane to be cured without interfering with the coating, and the pressure sensitive adhesive layer 13 can thus be formed.

The addition-type organopolysiloxane can contain an organopolysiloxane (silicone resin) that contains a trifunctional or tetrafunctional siloxane unit in the molecule in order to increase the adhesive strength.

The content of the organopolysiloxane, which contains a trifunctional or tetrafunctional siloxane unit, in the above silicone-based pressure sensitive adhesive may be preferably 0-100 mass parts, particularly preferably 5-70 mass parts, and further preferably 10-50 mass parts to 100 mass parts of the addition-type organopolysiloxane.

(1-3) Rubber-based Pressure Sensitive Adhesive

The rubber-based pressure sensitive adhesive may preferably contain an A-B-A-type block copolymer as a rubber elastic component and a tackifier and, if desired, further contain an antioxidant for preventing deterioration.

Examples of the A-B-A-type block copolymer include a styrene-isoprene-styrene copolymer, styrene-butylene-styrene copolymer, styrene-ethylene-butylene-styrene copolymer, styrene-olefin-styrene copolymer, polyisoprene, polybutene, and polyisobutylene.

The content of the rubber elastic component in the rubber-based pressure sensitive adhesive may be preferably 5-50 mass % and particularly preferably 7-45 mass %. When the content of the rubber elastic component is 5 mass % or more, the cohesive strength of the rubber-based pressure sensitive adhesive can be maintained high, while when the content of the rubber elastic component is 50 mass % or less, the adhesive strength of the pressure sensitive adhesive layer can be moderately high.

Examples of the tackifier include a rosin-based resin, polyterpene-based resin, coumarone/indene resin, petroleum-based resin, terpene phenol resin, alkylphenol resin, styrene-based resin, phenol-based resin, and xylene resin.

The content of the tackifier in the rubber-based pressure sensitive adhesive may be preferably 10-70 mass % and particularly preferably 15-60 mass %. If the content of the tackifier is less than 10 mass %, the adhesive strength of the pressure sensitive adhesive layer decreases, while if the content of the tackifier exceeds 70 mass %, the cohesive strength of the rubber-based pressure sensitive adhesive may be unduly low.

Examples of the antioxidant include butylhydroxytoluene, 2,6-di-t-butyl-4-methylphenol, 2,5-di-t-butylhydroquinone, mercaptobenzimidazole, 1,1-bis(4-hydroxyphenol)cyclohexane, and phenyl-p-naphthylamine.

When an antioxidant is used, the content of the antioxidant in the rubber-based pressure sensitive adhesive may be preferably 0.1-10 mass % and particularly preferably 0.2-5 mass %.

(2) Physical Properties of Pressure Sensitive Adhesive/Pressure Sensitive Adhesive Layer

In the pressure sensitive adhesive sheet 1A, 1B, 1C according to the present embodiment, the storage elastic modulus at 120° C. of the pressure sensitive adhesive layer 13 may be preferably 1-500 kPa, particularly preferably 10-200 kPa, and further preferably 40-100 kPa. When the storage elastic modulus is within the above range, the pressure sensitive adhesive layer 13 has sufficient hardness even under a condition of relatively high temperatures. Thus, even when pressure is applied to the pressure sensitive adhesive sheet 1A, 1B, 1C, the pressure sensitive adhesive layer 13 is less likely to deform and protrude from the base material 11 and the inorganic filler-containing resin layer 12. This suppresses the occurrence of troubles in an electronic component due to the pressure sensitive adhesive layer 13. A method of measuring the storage elastic modulus at 120° C. of the pressure sensitive adhesive layer 13 is as described in the exemplary test, which will be described later.

In the pressure sensitive adhesive sheet 1A, 1B, 1C according to the present embodiment, the gel fraction of the pressure sensitive adhesive which constitutes the pressure sensitive adhesive layer 13 may preferably be 20-100%. In particular, when the pressure sensitive adhesive which constitutes the pressure sensitive adhesive layer 13 is an acrylic-based pressure sensitive adhesive, the gel fraction may be preferably 20-98%, particularly preferably 5 0-95%, and further preferably 70-85%. The gel fraction being 20% or more allows the pressure sensitive adhesive to have sufficient cohesive strength. On the other hand, the gel fraction being 98% or less allows the pressure sensitive adhesive sheet 1A, 1B, 1C to have moderate flexibility and, even when the pressure sensitive adhesive sheet 1A, 1B, 1C is attached to a surface having a height difference, the pressure sensitive adhesive sheet 1A, 1B, 1C can well follow the height difference. A method of measuring the gel fraction is as described in the exemplary test, which will be described later.

(3) Thickness of Pressure Sensitive Adhesive Layer

The thickness of the pressure sensitive adhesive layer 13 may be preferably 1 μm or more, particularly preferably 3 μm or more, and further preferably 4 μm or more. From another aspect, the thickness of the pressure sensitive adhesive layer 13 may be preferably 50 μm or less, particularly preferably 15 μm or less, and further preferably 9 μm or less. When the thickness of the pressure sensitive adhesive layer 13 is 1 μm or more, the pressure sensitive adhesive sheet 1A, 1B, 1C can exhibit sufficient adhesive strength. When the thickness of the pressure sensitive adhesive layer 13 is 50 μm or less, it is possible to prevent mixture or the like of the residual solvent and bubbles into the pressure sensitive adhesive layer 13 and easily adjust the above-described storage elastic modulus and the adhesive strength, which will be described later.

4. Release Sheet

The release sheet 14 is to protect the pressure sensitive adhesive layer until the use of the pressure sensitive adhesive sheet 1A, 1B, 1C and is removed when using the pressure sensitive adhesive sheet 1A, 1B, 1C. In the pressure sensitive adhesive sheet 1A, 1B, 1C according to the present embodiment, the release sheet 14 may not necessarily be required.

Examples of the release sheet 14 to be used include a polyethylene film, polypropylene film, polybutene film, polybutadiene film, polymethylpentene film, polyvinyl chloride film, vinyl chloride copolymer film, polyethylene terephthalate film, polyethylene naphthalate film, polybutylene terephthalate film, polyurethane film, ethylene-vinyl acetate film, ionomer resin film, ethylene-(meth)acrylic acid copolymer film, ethylene-(meth)acrylic ester copolymer film, polystyrene film, polycarbonate film, polyimide film, fluorine resin film, and liquid crystal polymer film. Crosslinked films thereof may also be used. A laminate film obtained by laminating a plurality of such films may also be used.

It may be preferred to perform release treatment for the release surface (surface to be in contact with the pressure sensitive adhesive layer 13) of the release sheet 14. Examples of a release agent to be used for the release treatment include alkyd-based, silicone-based, fluorine-based, unsaturated polyester-based, polyolefin-based, and wax-based release agents.

The thickness of the release sheet 14 is not particularly restricted, but may ordinarily be about 20-150 μm.

5. Physical Properties Etc. of Pressure Sensitive Adhesive Sheet

The adhesive strength of the pressure sensitive adhesive sheet 1A, 1B, 1C according to the present embodiment to an aluminum plate may be preferably 0,1-20 N/25 mm, particularly preferably 0.5-10 N/25 mm, and further preferably 1-5 N/25 mm. As used herein, the adhesive strength refers basically to a peel strength that is measured using a method of 180° peeling in accordance with JIS Z0237: 2009. Details of the method of measurement are as described in the exemplary test, which will be described later.

The thickness of the pressure sensitive adhesive sheet 1A, 1B, 1C without the release sheet may be preferably 5-250 μm, particularly preferably 10-110 μm, and further preferably 15-45 μm.

<Method of Manufacturing Pressure Sensitive Adhesive Sheet>

The pressure sensitive adhesive sheet 1A according to the present embodiment can be manufactured, for example, through producing a laminate of the base material 11 and the inorganic filler-containing resin layer 12 and a laminate of the pressure sensitive adhesive layer 13 and the release sheet 14 and attaching these laminates to each other so that the inorganic filler-containing resin layer 12 comes into contact with the pressure sensitive adhesive layer 13.

The pressure sensitive adhesive sheet 1B according to the present embodiment can be manufactured, for example, through producing a laminate of the base material 11 and the inorganic filler-containing resin layer 12 and a laminate of the pressure sensitive adhesive layer 13 and the release sheet 14 and attaching these laminates to each other so that the base material 11 comes into contact with the pressure sensitive adhesive layer 13.

The pressure sensitive adhesive sheet 1C according to the present embodiment, can be manufactured, for example, through producing a laminate in which both surfaces of the base material 11 are formed with the inorganic filler-containing resin layers 12 and a laminate of the pressure sensitive adhesive layer 13 and the release sheet 14 and attaching these laminates to each other so that one of the inorganic filler-containing resin layers 12 comes into contact with the pressure sensitive adhesive layer 13.

From the viewpoint of enhancing the interfacial adhesion between the inorganic filler-containing resin layer 12 and the pressure sensitive adhesive layer 13, it may be preferred to perform surface treatment such as corona treatment and plasma treatment for a surface to be attached of any one of these layers or surfaces to be attached of both of these layers and then attach these layers to each other.

The laminate of the base material 11 and the inorganic filler-containing resin layer 12 can be produced, for example, in the following manner. First, one main surface of the base material 11 may be coated with a coating agent that contains the composition for inorganic filler-containing resin layer and may further contain a solvent if desired, and the coating agent may be dried. The method of coating with the coating agent may be performed using an ordinary method, such as a bar coating method, knife coating method, Meyer bar method, roll coating method, blade coating method, die coating method, and gravure coating method. Drying can be performed, for example, by heating at 80-150° C. for about 30 seconds to 5 minutes.

Thereafter, the layer obtained by drying the above coating agent may be irradiated with active energy rays to cure the layer to form the inorganic filler-containing resin layer 12. As the active energy rays, for example, electromagnetic wave or charged particle radiation having an energy quantum can be used and, specifically, ultraviolet rays, electron rays or the like can be used. In particular, ultraviolet rays may be preferred because of easy management. Irradiation with ultraviolet rays can be performed using a high pressure mercury lamp, xenon lamp or the like, and the irradiance level of ultraviolet, rays may be preferably about 50-1,000 mW/cm² as the illuminance. The light amount may be preferably 50-10,000 mJ/cm², more preferably 80-5,000 mJ/cm², and particularly preferably 200-2,000 mJ/cm². On the other hand, irradiation with electron rays can be performed using an electron ray accelerator or the like, and the irradiance level of electron rays may be preferably about 10-1,000 krad.

The laminate of the pressure sensitive adhesive layer 13 and the release sheet 14 can be produced, for example, in the following manner.

When the pressure sensitive adhesive layer 13 is formed using an acrylic-based pressure sensitive adhesive, the release surface of the release sheet 14 may be coated with a coating solution that contains the above-described acrylic-based pressure sensitive adhesive and may further contain a solvent if desired, and heating treatment may be performed to form a coating film. The formed coating film itself may be the pressure sensitive adhesive layer 13 if an aging period is not necessary. If the aging period is necessary, the formed coating film may become the pressure sensitive adhesive layer 13 after the aging period passes.

Drying treatment when volatilizing a diluent solvent and the like of the coating solution can also serve as the above heating treatment. When performing the heating treatment, the heating temperature may be preferably 50-150° C. and particularly preferably 70-120° C. The heating time may be preferably 30 seconds to 10 minutes and particularly preferably 50 seconds to 2 minutes. If necessary, an aging period may be provided after the heating treatment at an ordinary temperature (e.g. 23° C. and 50% RH) for one to two weeks. When the acrylic-based pressure sensitive adhesive contains a (meth)acrylic ester polymer having a reactive functional group and a crosslinker, the heating treatment allows a structure to be formed in which the (meth)acrylic ester polymer is crosslinked by the crosslinker.

When the pressure sensitive adhesive layer 13 is formed using a silicone-based pressure sensitive adhesive, the release surface of the release sheet 14 may be coated with a coating solution, which is then heated for curing to form the pressure sensitive adhesive layer 13. The coating solution may be obtained, for example, by diluting an addition-type organopolysiloxane and a platinum catalyst and if desired an organopolysiloxane that contains a trifunctional or tetrafunctional siloxane unit, to about 10-60 mass % using a solvent such as methyl ethyl ketone, toluene, ethyl acetate, and xylene. The heating temperature may preferably be about 90-180° C. and the heating time may preferably be about 1-5 minutes

Another method of manufacturing the pressure sensitive adhesive sheet 1A according to the present embodiment may include forming the inorganic filler-containing resin layer 12 and the pressure sensitive adhesive layer 13 in this order on the base material 11. Another method of manufacturing the pressure sensitive adhesive sheet 1B according to the present embodiment may include forming the inorganic filler-containing resin layer 12 and the pressure sensitive adhesive layer 13 on respective surfaces of the base material 11. Another method of manufacturing the pressure sensitive adhesive sheet 1C according to the present embodiment may include forming the inorganic filler-containing resin layers 12 on both surfaces of the base material 11 and then laminating the pressure sensitive adhesive layer 13 on one of the inorganic filler-containing resin layers 12.

<Method of using Pressure Sensitive Adhesive Sheet>

The pressure sensitive adhesive sheet 1A, 1B, 1C according to the present embodiment can be preferably used for an electronic component. Examples of the electronic component include a flexible printed board, battery, rigid board, and motor. In particular, the pressure sensitive adhesive sheet 1A, 1B, 1C can be used in an electronic component in a state of being in contact with a conductor through which a current flows. In this case, the surface at the pressure sensitive adhesive layer 13 side of the pressure sensitive adhesive sheet 1A, 1B, 1C may be attached to the conductor, or the surface at the base material 11 side may be brought into contact with the conductor. In an embodiment, both surfaces of the pressure sensitive adhesive sheet 1A, 1B, 1C may be brought into contact with conductors. The pressure sensitive adhesive sheet 1A, 1B, 1C can be used, for example, for the purpose of protecting a conductor such as an electric circuit in an electronic component. The pressure sensitive adhesive sheet 1A, 1B, 1C can also be used for the purpose of fixing a conductor such as an electrode in an electronic component.

The pressure sensitive adhesive sheet 1A, 1B, 1C according to the present embodiment is provided with the base material 11 having insulation properties and thereby has good insulation properties at an ordinary time. Moreover, even when the matrices in the base material 11 and/or the inorganic filler-containing resin layer 12 undergo carbonization and/or combustion/decomposition due to high temperatures, the insulation properties of the pressure sensitive adhesive sheet 1A, 1B, 1C itself are ensured because the inorganic filler in the inorganic filler-containing resin layer 12 remains as an insulating film. Furthermore, even when a high voltage is partially applied to the pressure sensitive adhesive sheet 1A, 1B, 1C to flow a current through the pressure sensitive adhesive sheet 1A, 1B, 1C in the thickness direction so that the path of the current in the base material 11 changes to a conductor and the matrix in the inorganic filler-containing resin layer 12 undergoes carbonization and the like along the path of the current, the insulation properties of the pressure sensitive adhesive sheet 1A, 1B, 1C itself are ensured because the inorganic filler exists as an insulating body in the path. Owing to these features, the occurrence of short circuit and/or thermal runaway is suppressed in an electronic component for which the pressure sensitive adhesive sheet 1A, 1B, 1C is used.

It should be appreciated that the embodiments heretofore explained are described to facilitate understanding of the present invention and are not described to limit the present invention. It is therefore intended that the elements disclosed in the above embodiments include all design changes and equivalents to fall within the technical scope of the present invention.

For example, in the pressure sensitive adhesive sheet 1A, 1B, 1C, the release sheet 14 may be omitted. In an embodiment, the pressure sensitive adhesive sheet 1A, 1B, 1C may be provided with one or more other layers between the base material 11 and the inorganic filler-containing resin layer 12.

EXAMPLES

Hereinafter, the present invention will be described further specifically with reference to examples, etc., but the scope of the present invention is not limited to these examples, etc.

Example 1 1. Formation of Inorganic Filler-Containing Resin Layer on Base Material

A coating liquid for inorganic filler-containing resin layer was prepared through mixing 40 mass parts of dipentaerythritol hexaacrylate (a material of which the glass-transition point is not observed after curing) as an active energy ray-curable component, 5 mass parts of hydroxycyclohexyl phenyl ketone as a photopolymerization initiator, and 60 mass parts (solid content equivalent, here and hereinafter) of an organosilica sol (available from Nissan Chemical Industries, Ltd., trade name “MEK-ST,” average particle diameter of 30 nm) as an inorganic filler and diluting them with methyl ethyl ketone. The solid content concentration of the coating liquid for inorganic filler-containing resin layer was 20 mass %.

One surface of a polyimide film (available from DU PONT-TORAY CO., LTD., trade name “Kapton 100H,” thickness of 25 nm, flame retardation level V-0 of UL94 standard) as a base material was coated with the above coating liquid using a knife coater and the coating liquid was then dried at 70° C. for 1 minute. Subsequently, the coating film was irradiated with ultraviolet rays (illuminance of 230 mW/cm2, light amount of 510 mJ/cm2) to cure the coating film. A first laminate was thus obtained in which an inorganic filler-containing resin layer having a thickness of 2 μm and containing 2.0 g/m² of the inorganic filler was formed on one surface of the base material.

2. Formation of Pressure Sensitive Adhesive Layer on Release Sheet

A (meth)acrylic ester polymer was prepared by copolymerizing 77 mass parts of butyl acrylate, 20 mass parts of methyl acrylate, and 3 mass parts of acrylic acid. The molecular weight of this polymer was measured using gel permeation chromatography (GPC), which will be described later. The weight-average molecular weight (Mw) was 900,000. Then, a coating liquid for pressure sensitive adhesive layer was prepared through mixing 100 mass parts of the (meth)acrylic ester polymer, 2,2 mass parts of trimethylolpropane-modified tolylene diisocyanate (available from TOYOCHEM CO., LTD., trade name “BHS8515”) as a crosslinker, and 0.3 mass parts of aluminum tris(acetylacetonate) (available from Soken Chemical & Engineering Co., Ltd., trade name “M-5A”) as a crosslinker and diluting them with methyl, ethyl, ketone. The solid content concentration of the coating liquid for pressure sensitive adhesive layer was 20 mass %.

A release sheet (available from LINTEC Corporation, trade name “PET251130”) was prepared in which one surface of a polyethylene terephthalate film was subjected to release treatment using a silicone-based release agent. The release-treated surface of the release sheet was coated with the obtained coating liquid using a knife coater and the coating liquid was then heat-treated at 120° C. for 1minute. A second laminate was thus obtained in which a pressure sensitive adhesive layer composed of the acrylic-based pressure sensitive adhesive and having a thickness of 5 μm was laminated on the release-treated surface of the release sheet.

3. Production of Pressure Sensitive Adhesive Sheet

The surface at the inorganic filler-containing resin layer side of the first laminate produced as the above and the surface at the pressure sensitive adhesive layer side of the second laminate produced as the above were attached to each other, and a pressure sensitive adhesive sheet was thus obtained.

Example 2

A coating liquid for inorganic filler-containing resin layer was prepared through mixing 45 mass parts of a polyamideimide resin (available from “TOYOBO CO., LTD., trade name “VYLOMAX CHX02”) and 60 mass parts of an organosilica sol (available from Nissan Chemical Industries, Ltd., trade name “MEK-ST”) as an inorganic filler and diluting them with methyl ethyl ketone. The solid content concentration of the coating liquid for inorganic filler-containing resin layer was 20 mass %. A pressure sensitive adhesive sheet was manufactured in the same manner as in Example 1 except that the coating liquid for inorganic filler-containing resin layer was used. In this pressure sensitive adhesive sheet, the content of the inorganic filler in the inorganic filler-containing resin layer was 1.2 g/m².

Example 3

A coating liquid for pressure sensitive adhesive layer was prepared through mixing 100 mass parts of a silicone-based pressure sensitive adhesive (available from Dow Corning Toray Co., Ltd., trade name “SD-4584”) and 0.5 mass parts of a catalyst (available from Dow Corning Toray Co., Ltd., trade name “CAT-SRX-212”) and diluting them with methyl ethyl ketone. The solid content concentration of the coating liquid for pressure sensitive adhesive layer was 20 mass %. A pressure sensitive adhesive sheet was manufactured in the same manner as in Example 1 except that the coating liquid for pressure sensitive adhesive layer and a release sheet (available from LINTEC Corporation, trade name “PET50FD”) obtained by release-treating one surface of a polyethylene terephthalate film using a fluorine-based release agent were used.

Example 4

A coating liquid for pressure sensitive adhesive layer was prepared through mixing 100 mass parts of polyisobutylene (available from JAPAN BUTYL Co. Ltd., trade name “Exxon Butyl 268”), 5 mass parts of a methacryloyl group-containing polyisoprene rubber (available from KURARAY CO., LTD., trade name “UC-203”), and 20 mass parts of an aliphatic-based petroleum resin (available from Zeon Corporation, trade name “Quintone A100”) and diluting them with toluene. The solid content concentration of the coating liquid for pressure sensitive adhesive layer was 20 mass %. A pressure sensitive adhesive sheet was manufactured in the same manner as in Example 1 except that the coating liquid for pressure sensitive adhesive layer was used.

Example 5

A, coating liquid for inorganic filler-containing resin layer was prepared through mixing 40 mass parts of dipentaerythritol hexaacrylate as an active energy ray-curable component, 5 mass parts of hydroxycyclohexyl phenyl ketone as a photopolymerization initiator, and 60 mass parts of alumina ceramic nanoparticles (available from BYK Japan KK, trade name “NANGBYK-3601”) as an inorganic filler and diluting them with methyl ethyl ketone. The solid content concentration of the coating liquid for inorganic filler-containing resin layer was 20 mass %. A pressure sensitive adhesive sheet was manufactured in the same manner as in Example 1 except that the coating liquid for inorganic filler-containing resin layer was used. In this pressure sensitive adhesive sheet, the content of the inorganic filler in the inorganic filler-containing resin layer was 2.2 g/m².

Example 6

A (meth)acrylic ester polymer was prepared by copolymerizing 60 mass parts of butyl acrylate, 20 mass parts of methyl acrylate, and 20 mass parts of acrylic acid. The molecular weight of this polymer was measured using gel permeation chromatography (GPC), which will be described later. The weight-average molecular weight (Mw) was 600,000. A coating liquid for pressure sensitive adhesive layer was prepared through mixing 100 mass parts of the (meth)acrylic ester polymer and 2.2 mass parts of trimethylolpropane-modified tolylene diisocyanate (available from TOYOCHEM CO., LTD., trade name “BHS8515”) as a crosslinker and diluting them with methyl ethyl ketone. The solid content concentration of the coating liquid for pressure sensitive adhesive layer was 20 mass %. A pressure sensitive adhesive sheet was manufactured in the same manner as in Example 1 except that the coating liquid for pressure sensitive adhesive layer was used.

Example 7

A (meth)acrylic ester polymer was prepared by copolymerizing 99 mass parts of butyl acrylate and 1 mass part of 4-hydroxybutyl (meth)acrylate. The molecular weight of this polymer was measured using gel permeation chromatography (GPC), which will be described later. The weight-average molecular weight (Mw) was 1,500,000. A coating liquid for pressure sensitive adhesive layer was prepared through mixing 100 mass parts of the (meth)acrylic ester polymer and 2.2 mass parts of trimethylolpropane-modified tolylene diisocyanate (available from TOYOCHEM CO., LTD., trade name “BHS8515”) as a crosslinker and diluting them with methyl ethyl ketone. The solid content concentration of the coating liquid for pressure sensitive adhesive layer was 20 mass %. A pressure sensitive adhesive sheet was manufactured in the same manner as in Example 1 except that the coating liquid for pressure sensitive adhesive layer was used.

Example 8

A coating liquid for inorganic filler-containing resin layer was prepared through mixing 40 mass parts of dipentaerythritol hexaacrylate as an active energy ray-curable component, 5 mass parts of hydroxycyclohexyl phenyl ketone as a photopolymerization initiator, and 60 mass parts of reactive silica (silica fine particles having acryloyl groups on the surfaces, average particle diameter of 15 nm before surface modification of silica fine particles) as an inorganic filler and diluting them with methyl ethyl ketone. The solid content concentration of the coating liquid for inorganic filler-containing resin layer was 20 mass %. A pressure sensitive adhesive sheet was manufactured in the same manner as in Example 1 except that the coating liquid for inorganic filler-containing resin layer was used. In this pressure sensitive adhesive sheet, the content of the inorganic filler in the inorganic filler-containing resin layer was 2.0 g/m².

Example 9

A pressure sensitive adhesive sheet was manufactured in the same manner as in Example 1 except that the surface at the base material side of the first laminate and the surface at the pressure sensitive adhesive layer side of the second laminate were attached to each other.

Example 10

A pressure sensitive adhesive sheet was manufactured in the same manner as in Example 2 except that the surface at the base material side of the first laminate and the surface at the pressure sensitive adhesive layer side of the second laminate were attached to each other,

Comparative Example 1

A pressure sensitive adhesive sheet was manufactured in the same manner as in Example 1 except that a polyimide film (available from DU PONT-TORAY CO., LTD., trade name “Kapton 100H,” thickness of 25 μm) as a base material was used as substitute for the first laminate and one surface of the base film and the surface at the pressure sensitive layer side of the second laminate were attached to each other.

<Exemplary Test 1>(Measurement of Adhesive Strength)

The adhesive strength of the pressure sensitive adhesive sheets was measured in accordance with JIS Z0237: 2009 except the following operation.

The pressure sensitive adhesive sheet obtained in each of the examples or the comparative example was cut into a width of 25 mm and a length of 250 mm and the release sheet was then removed to obtain a test piece. The exposed pressure sensitive adhesive layer of the test piece was attached to an aluminum plate as an adherend using a rubber roller of 2 kg under an environment of 23° C. and 50% RH and they were then left untouched under the same environment for 20 minutes. Thereafter, the test piece was peeled off from the above aluminum plate at a peel angle of 180° and a peel speed of 300 mm/min using a universal tensile tester (available from ORIENTEC Co., LTD., “TENSILON UTM-4-100”) and the peel strength (N/25 mm) was thus measured. The measured value was employed as the adhesive strength. Results are listed in Table 1.

<Exemplary Test 2>(Measurement of Insulation Breakdown Voltage)

For the base material used for manufacturing the pressure sensitive adhesive sheet in each of the examples or the comparative example, the minimum voltage at which insulation breakdown occurred (which may be referred to as “insulation breakdown voltage,” hereinafter) was measured in accordance with item 17 “Insulation Strength” of JIS C2107. Specifically, first, the base material was cut into 300 mm×300 mm to obtain a test piece. Then, both surfaces of the test piece were interposed between electrodes of a withstand voltage tester (available from Keisoku Giken Co., Ltd., trade name “AC withstand voltage tester 7220”). This setting was performed such that each electrode was in contact with the middle part of the test piece. Subsequently, the current was set at 10 mA and the voltage was increased at a rate of 500 V/s from 0 V until the insulation breakdown occurred. In this operation, the voltage at which insulation breakdown occurred was measured and recorded. Results are listed in Table 1.

<Exemplary Test 3>(Evaluation of Insulation Properties at Ordinary Time)

The release film (release sheet) was removed from the pressure sensitive adhesive sheet obtained in each of the examples or the comparative example and the exposed pressure sensitive adhesive layer was attached to an aluminum plate to obtain a test piece. Then, the resistance value between the surface at the pressure sensitive adhesive sheet side and the surface at the aluminum plate side of the test piece was measured using a digital high tester (available from HIOKI E.E. CORPORATION, trade name “3802-50”). Through this operation, the resistance value in the thickness direction of the pressure sensitive adhesive sheet was measured.

On the basis of the measured resistance value in the thickness direction of the pressure sensitive adhesive sheet, the insulating properties of the pressure sensitive adhesive sheet at an ordinary time were evaluated in accordance with the following determination criteria. Evaluation results are listed in Table 1.

∘: The resistance value in the thickness direction of the pressure sensitive adhesive sheet was 1.0×10¹⁰ Ω or more.

×: The resistance value in the thickness direction of the pressure sensitive adhesive sheet was less than 1.0×10¹⁰ Ω.

<Exemplary Test 4>(Evaluation of Insulation Properties After Heating)

The release film (release sheet) was removed from the pressure sensitive adhesive sheet obtained in each of the examples or the comparative example and the exposed pressure sensitive adhesive layer was attached to an aluminum plate to obtain a test piece. The test piece was heated under a nitrogen atmosphere environment of 800° C. for one hour. Then, the resistance value between the surface at the pressure sensitive adhesive sheet side and the surface at the aluminum plate side of the test piece was measured using a digital high tester (available from HIOKI E.E. CORPORATION, trade name “3802-50”). Through this operation, the resistance value in the thickness direction of the pressure sensitive adhesive sheet was measured,

On the basis of the measured resistance value in the thickness direction of the pressure sensitive adhesive sheet, the insulating properties of the pressure sensitive adhesive sheet after heating were evaluated in accordance with the following determination criteria. Evaluation results are listed in Table 1.

∘: The resistance value in the thickness direction of the pressure sensitive adhesive sheet was 1.0×10¹⁰ Ω or more.

×: The resistance value in the thickness direction of the pressure sensitive adhesive sheet was less than 1.0×10¹⁰ Ω.

<Exemplary Test 5>(Evaluation of Insulation Properties After Insulation Breakdown)

The release film (release sheet) was removed from, the pressure sensitive adhesive sheet obtained in each of the examples or the comparative example and the exposed pressure sensitive adhesive layer was attached to an aluminum plate to obtain a test piece. A voltage of 10 kV was applied to the test piece. Then, the resistance value between the surface at the pressure sensitive adhesive sheet side and the surface at the aluminum plate side of the test piece was measured using a digital high tester (available from HIOKI E.E, CORPORATION, trade name “3802-50”). Through this operation, the resistance value in the thickness direction of the pressure sensitive adhesive sheet was measured.

On the basis of the measured resistance value in the thickness direction of the pressure sensitive adhesive sheet, the insulating properties of the pressure sensitive adhesive sheet after insulation breakdown were evaluated in accordance with the following determination criteria. Evaluation results are listed in Table 1.

∘: The resistance value in the thickness direction of the pressure sensitive adhesive sheet was 1.0×10¹⁰ Ω or more.

×: The resistance value in the thickness direction of the pressure sensitive adhesive sheet was less than 1.0×10¹⁰ Ω.

<Exemplary Test 6>(Measurement of Gel Fraction)

In the manufacturing of the pressure sensitive adhesive sheet of each of the examples and comparative example, the surface at the pressure sensitive adhesive layer side of the second laminate was attached to the release-treated surface of another release sheet (available from LINTEC Corporation, trade name “PET251130”) in which one surface of a polyethylene terephthalate film was subjected to release treatment using a silicone-based release agent. A sheet for measurement was thus obtained comprising the pressure sensitive adhesive layer alone, of which both surfaces were protected by the release sheets.

The obtained sheet for measurement was cut into a size of 80 mm×80 mm and the release sheets protecting both surfaces of the pressure sensitive adhesive layer were removed. The pressure sensitive adhesive layer was wrapped with a polyester mesh (mesh size of 200) and the total mass was weighed using a precision balance. The mass of the pressure sensitive adhesive alone was calculated by subtracting the mass of the above mesh alone from the total mass. The calculated mass is represented by M1. Then, the pressure sensitive adhesive wrapped with the above polyester mesh was immersed in ethyl acetate at 25 ° C. for 24 hours. Thereafter, the pressure sensitive adhesive was taken out, air-dried for 24 hours under an environment of a temperature of 23° C. and a relative humidity of 50%, and further dried in an oven at 80° C. for 12 hours. After drying, the mass was weighed using a precision balance and the mass of the pressure sensitive adhesive alone was calculated by subtracting the mass of the above mesh alone. The calculated mass is represented by M2. The gel fraction (%) was calculated from the calculation formula of (M2/M1)×100. Results are listed in Table 1.

<Exemplary Test 7>(Measurement of Storage Elastic Modulus>

The release sheet was removed from, the second laminate obtained in each of the examples or the comparative example to expose the pressure sensitive adhesive layer. A plurality of such pressure sensitive adhesive layers was laminated so as to have a thickness of 0.6 mm. A cylindrical body having a diameter of 8 mm (height of 0.6 mm) was punched out from the obtained laminate of the pressure sensitive adhesive layers and this was employed as a sample.

The storage elastic modulus (MPa) of the above sample was measured by a torsional shear testing method under the following condition in accordance with JTS K7244-6 using a viscoelasticity measuring apparatus (available from Physica, MCR300). Measurement results are listed in Table 1.

Measurement frequency: 1 Hz

Measurement temperature: 120° C.

<Exemplary Test 8>(Evaluation of Pencil Hardness)

For the inorganic filler-containing resin layer of the first laminate obtained when manufacturing the pressure sensitive adhesive sheet of each of the examples and comparative example, the pencil hardness was measured in accordance with JIS K5600-5-4. A pencil scratch hardness tester (available from YASUDA SEIKI SEISAKUSHO, LTD., trade name “No. 553-M”) was used for the measurement and the scratch speed was 1 mm/sec. Results are listed in Table 1.

TABLE 1 Example Comparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Example 8 Example 9 10 Example 1 Adhesive  5  5 10  6  5  7  4  5  5  6  5 strength (N/25 mm) Insulation  9  9 10  9  9  9  9  9  9  9  7 breakdown voltage (kV) Evaluation of ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ insulation properties at ordinary time Evaluation of ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ X insulation properties after heating Evaluation of ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ X insulation properties after insulation breakdown Gel fraction 80 80 90 100  80 75 85 80 80 80 80 (%) Storage elastic 72 72 80 93 72 26 58 72 72 72 72 modulus at 120° C. (kPa) Pencil HB 2B HB HB HB HB HB H HB 2B 2B hardness of inorganic filler- containing resin layer

As apparent from. Table 1, the pressure sensitive adhesive sheets of the examples exhibit, good, insulation properties even after they are exposed to high temperatures and insulation breakdown is caused, which are different from those of the pressure sensitive adhesive sheet of the comparative example.

INDUSTRIAL APPLICABILITY

The pressure sensitive adhesive sheet of the present invention is suitable as a pressure sensitive adhesive sheet that is used in electronic components.

DESCRIPTION OF REFERENCE NUMERALS

1A, 1B, 1C . . . Pressure sensitive adhesive sheet

11 . . . Base material

12 . . . Inorganic filler-containing resin layer

13 . . . Pressure sensitive adhesive layer

14 . . . Release sheet 

1. A pressure sensitive adhesive sheet comprising: a base material having insulation properties; an inorganic filler-containing resin layer provided at least at one side of the base material and containing an insulating inorganic filler; and a pressure sensitive adhesive layer provided as an outermost layer of at least one surface.
 2. The pressure sensitive adhesive sheet as recited in claim 1, wherein the base material has flame retardancy that satisfies flame retardancy level V-0 of UL94 standard.
 3. The pressure sensitive adhesive sheet as recited in claim 1, wherein the inorganic filler-containing resin layer is provided at the one side of the base material and the pressure sensitive adhesive layer is provided at a side of the inorganic filler-containing resin layer opposite to the base material.
 4. The pressure sensitive adhesive sheet as recited in claim 1, wherein a content of the inorganic filler in the inorganic filler-containing resin layer is 0.5-50 g/m².
 5. The pressure sensitive adhesive sheet as recited in claim 1, wherein the inorganic filler-containing resin layer comprises a material obtained by curing a composition that contains an active energy ray-curable component and the inorganic filler.
 6. The pressure sensitive adhesive sheet as recited in claim 1, wherein a minimum voltage at which insulation breakdown occurs in the base material is 1 kV or higher.
 7. The pressure sensitive adhesive sheet as recited in claim 1, wherein the pressure sensitive adhesive sheet is to be used in an electronic component. 